Based on some apps I've seen there are some options. The option I describe below uses a progressive-disclosure-like method of account creation, meaning they add to their account info over time.
Basically, ask for something simple like a phone number. Phone numbers are easy to enter, the number keyboard has large buttons. If you need their phone number anyway, start with this. The con is, if they don't understand why you want their phone number, this could be a battle.
If not a phone number, just ask for something you need, but nothing they have to type. Something they can just tap or click on. Have them input their gender or birthdate (if you need those things). It's easy to just tap on things. It just needs to be clear why you're asking.
Once you have their initial piece of information, allow them to access the application. Give them empty placeholders in their account to fill in later. You can even use alerts or local notifications to ask for additional pieces of information over time. You can lock down certain features by telling the user they must have an account to use them, at which point they would go in and finish it all at once (now they are motivated).
Also, get as much of their info in context as possible. If you need their location, for example, ask for it when they attempt to use a feature that requires location. If you need their credit card number, wait until they are trying to purchase something. Not knowing much about your app it's hard to tell you exactly what to ask for, but you can take this and customize it for your need.
Doing it this way might take some back-end work, like implementing the ability to create a user account based on a single piece of information. But it's worth the benefits of improved onboarding and higher sign-up rates. And you'll have to explore whether it's possible on the web. It might be tricky to allow them to access their account from multiple devices without anything to identify them uniquely. So the phone number route might be the best way. They'll just have to enter a verification code.
Check out Firebase Authentication. They allow you to track user accounts without having the user sign up. When the user finally does sign up, you can merge their anonymous data with their new account. Even if you don't use Firebase, it could give you some ideas.
Also, check out the app Tomorrow, which is an estate-planning app that has a pretty good onboarding experience.
